Kenors, that's the exact point, and the problem. The damn thing will lock up, no problem. However, up to that point, theres very little feel, or feedback. If a person moves up to a pad with more power, you are able to dial in some feedback before the lock up point.
As a reference point, if a person looks at aftermarket pads, the lowest rated I know of is that Galfer pad, rated "GG" in this scale: The brake pads sold in the usa HAVE to use this code. I think that's why we have trouble finding Brembo pads?
DOT Pad Codes
It's been a long time since I removed mine. As I recall, Kawasaki spaced "DF" rear pad. So little friction, it's not even on this scale and unless you ordered from Kawasaki, you wont find a pad that's that weak. The vesrah ones are stronger, but not as strong as the "HH" rated pad ebc sells.
Between that hard pad, and the heavy return spring, it's not very good for feel.
Rikifumi, for sure that spring has a purpose. If going without bothers anyone, find a smaller, less stiff spring from the hardware store. I believe Kawasaki went about 10x stiffer than needed just to be sure we dont lock the rear brake. Removing that spring is a band aid for what's really wrong, but it helps. Give it a try. You'll see the pedal returns just like it always did. You have to google something like " ninja 1000 weak rear brake" Theres stuff up there from 2012, on riderforum.
Steny, It's really worth looking at the rear brake before you do anything. Go out to the bike, and shine a flashlight into the bottom of the rear master cylinder. Notice how far you have to push the rear brake pedal to make it even start to depress the master cylinder. Think about how many times you pushed it, just a little, and never even overcame the play, and the strong spring.